Planetary drives are coaxial drives which are primarily constructed as harmonic drives (hereinafter HD drives) or as sliding wedge drives (hereinafter GK drives). The basic construction of HD drives is known from the American Magazine, Machine Design. 1960, pages 160-173" which has an internally toothed hollow wheel and a substantially cylindrical, thin-walled, externally toothed planetary wheel, which is referred to as the "flexspline". With the same tooth distribution, the aforementioned flexible planetary wheel has two fewer teeth than the hollow wheel. The planetary wheel further is provided with internal teeth with which an externally toothed sun wheel engages. A cam is provided which is formed like an ellipse and acts upon the flexible planetary wheel through a correspondingly deformable roller bearing in such a way that directly opposite a number of the teeth of the planetary wheel engage with the teeth of the hollow wheel. The teeth of the planetary wheel are brought into engagement displaced at an angle with the sun wheel on the one hand and with the hollow wheel on the other hand by a drive cam. The drive cam is a component of the plate-form cam wheel which is arranged adjacent the sun wheel spaced axially at the side and acts one-sidedly. The forces act in axially-spaced planes, and above all, the planetary wheel is subjected to high stresses and requires a high manufacturing cost. The planetary wheel is deformed not only in the radial direction, but instead as a result of the axial spacing of the engagement planes of the gear wheels, additional stresses result which adversely affect the operating life and functional reliability of the drive mechanism. At the very least after a rotation of 45.degree., the teeth in each case come out of engagement, and due to the elliptical configuration of the drive cam, or generally of the rotating body, a rolling down or shifting off occurs in the region of engagement. The HD drive makes possible reductions in the meaningful range of i=1:70 to i=1:300. With decreasing reductions, the number of the supporting teeth becomes smaller, and consequently the transmissible torque also decreases. The degree of ovalization of the planetary wheel, i.e. the ratio of the diameter at maximum deformation compared to the diameter at zero deformation, is large. In addition, the high manufacturing cost required to achieve a structure in which there is little play is disadvantageous.
Further the HD drive may be constructed as a flat drive which is kinematically similar in construction and contains a second hollow wheel for supporting the rotational torque. This second hollow wheel has the same number of teeth as the flexible planetary wheel, whereby however, the distribution of the teeth is larger so that the rotational torque must be reduced. Due to the limitation of the reduction ratio at approximately i=1:70 and further to the high manufacturing cost required to achieve high precision, use of HD drives of this type has been limited to robots, machine tools, and the like.
The construction of a planetary drive referred to as a GK drive is disclosed in U.S. Pat. No. 4,099,427. Two wheels with internal or external gear teeth are arranged adjacent each other with respect to a common axis, whereby there is a difference .DELTA.Z of from 2 to 4 in the number of teeth. Viewed in the axial direction, the teeth of the two gear wheels overlap so that so-called virtual tooth gaps are created, which move out of the teeth after a rotation of 180.degree. when there is a difference of two in the number of teeth and after a rotation of 90.degree. when there is a difference of 4 in the number of teeth. The tooth distribution and the flank angle thereby vary depending on the drive ratio and the depth of the teeth. A flexible planetary wheel engages radially in the virtual tooth gaps, whereby a flat contact exists at the flanks on both sides. In comparison with the initially described HD drives, additional degrees of freedom are provided with respect to the changes in tooth distribution and flank angle. The planetary wheel may be simply constructed in the form of a zig-zag formed, toothed band which accommodates itself to the changes of flank angle and tooth distribution. The toothed wheel and the planetary wheel each have a different tooth distribution. Further, an accommodation can take place by forming the teeth as bar teeth, pivotable teeth or bar teeth with a slight curvature. The teeth of the flexible planetary wheel force themselves into the virtual tooth gaps of the two axially adjacent gear wheels like wedges with flank contact on both sides, and indeed at a constant radial speed. The form of the rotating body or the drive cam is prescribed by semicircular sectors which are separated from the point of rotation by amounts .DELTA.x and .DELTA.y. For large reductions, additional degrees of freedom of the flexible planetary wheel may be omitted; in a prescribed region of engagement an average tooth distribution and an average flank angle are prescribed. The number of teeth engaging with each other is substantially independent of the drive ratio and is determined by the selected degree of freedom so that up to 60% of the teeth may be in engagement with each other. With a GK drive, rotational torques can be transmitted which are many times greater than can be transmitted with other types of construction. Particularly with a tooth number difference of four, the GK drive is free of play so that no rolling movements occur to speak of.
As a result of the two-sided surface contact, the teeth of the planetary wheel must be decoupled when the direction of movement is reversed so that they do not have any two-sided surface contact, for example, in the direction of the Y axis. The theoretical load bearing capacity is thereby reduced, and high production tolerances are required to achieve high proportions of load bearing teeth, particularly with bar teeth. For a tooth number difference of four, reduction ratios up to i =1:20 can be achieved, whereby however, a comparatively large deformation of the flexible planetary wheel takes place. On the other hand, the GK drive permits reduction ratios up to i=300. Reduction ratios larger or smaller than the aforementioned values can be achieved by construction as a three plate drive with two independently driven, flexible planetary wheels. Further, disadvantages may arise with GK drives in that the teeth of the planetary wheel are subjected to shear stress in the center; also, due to deformations which occur primarily at high rotational moments, engagement disturbances may result therefrom.
Planetary drives of both of the described types further have the common disadvantage that an unavoidable micro-movement takes place in the region of cylindrical contact between the flexible planetary wheel and the outer diameter of the roller bearing. Due to phase compression and stretching as well as to play which cannot be entirely avoided, a slippage occurs in the aforementioned region which leads to dry friction. Abrasive wear can be avoided only by the highest quality surfaces. The HD drive has a large axial structural length, and the HD flat drive, for kinematic reasons, serves to transmit relatively small rotational moments. Both types require a high construction cost when they are built into production machinery, such as, for example, industrial robots, machine tools, or motor drives. This high construction expense for flanges, drive shafts, supplementary housings, and bearings adds additional mass, which in practice is larger by a factor of three to four than the mass of the planetary drive. The competitiveness of such drives in comparison to multishaft and multi-stage toothed spur wheel planetary drives is thereby adversely affected. This additional mass leads to significant difficulties, particularly in industrial robots which make rapid adjusting movements.